Ion implantation includes ionizing, isolating and accelerating dopant atoms and sweeping an ion beam including the ionized dopant atoms across a wafer surface. The dopant ions enter the wafer and come to rest below the wafer surface. The depth the implanted ions reach in the wafer is a function of the incoming energy of the ions which are slowed in the wafer by electronic interaction and by physical collision with the host atoms in the wafer. The implanted ions are centered around an end-of-range peak. The ions damage the crystal lattice in a section of the wafer they pass by colliding with host atoms and by displacing the concerned host atoms from their lattice sites. Implanted dopant atoms that do not occupy regular lattice sites are electrically inactive and have no impact on the electric characteristics of the substrate. Typically, heating treatments restore the crystal lattice and electrically activate the dopant atoms by shifting them to regular lattice sites.
Conventional furnace heating techniques, for example, RTP (rapid thermal processing) affect all structures previously formed in the wafer. An LTA (laser thermal anneal) directly heats only a section of the semiconductor crystal and has less impact on previously formed structures in the wafer in a distance to the heated section.
In an LTA, absorption depth depends on the wavelength of the laser beam. Absorption depth of a laser beam with a wavelength of 308 nm is typically 10 nm and a melting depth down to which the 308 nm laser beam melts crystalline silicon is in a range up to about 500 nm. Melting depth can be extended to some degree by using antireflective coatings such that more energy of the laser beam is coupled into the semiconductor substrate. There is a need to improve methods for activating implanted dopants.